When heating an object using radiation from a heating source the temperature of the object is currently controlled and measured using pyrometers. Prior art is illustrated e.g. by U.S. Pat. No. 4,755,654; U.S. Pat. No. 4,919,542; U.S. Pat. No. 4,649,261; DE-A-4012615 and F. Roozeboom, Journal Vac. Sc. Techn., B8, 1249 (1990). The temperature measurement provided by pyrometers, however, is highly dependent on the emissivity of the object being heated and therefore measurement of the emissivity is desirable when accurate temperature measurement is required. However, measurement of the emissivity generally cannot be carried out efficiently outside the heating apparatus, i.e. at ambient temperature because under 600.degree. C. the emissivity is depending on the temperature level (P. Vandenabeele et al, Proc. MRS 224, 185, FIG. 4 (1990)). Consequently, it is necessary in practice to provide for in situ measurement of the emissivity above 600.degree. C.
For that purpose, methods have been provided which use external radiation sources arranged for intermittently projecting radiation onto the object to be heated and provide measurement of the reflected radiation from said object. The following documents are cited as prior art references : U.S. Pat. No. 4,956,538; U.S. Pat. No. 4,919,542; U.S. Pat. No. 4,979,133; U.S. Pat. No. 4,890,245; NL-A-8701479; DE-A-2153077. The drawback of these known methods is that the radiation source when used inside a heating chamber has a very narrow angular field of action whereby the emissivity measurement is greatly depending on the roughness of the surface of the object being irradiated. This drawback prevents these known methods from being applied generally (Mosledi et al, Proc. MRS 224, 143, (1990)).
In another known method an extended black body is heated to the same temperature as the object to be heated (U.S. Pat. No. 3,196,690; U.S. Pat. No. 3,969,943; GB-A-2,078,767; DE-A-3,422,590). The radiation from the black body is directed to the object; when the temperature of the black body is equal to the temperature of the object, the radiation from the black body is equal to the sum of the radiation from the object and the reflected radiation from the black body, whereby the temperature measurement of the object is independent from the emissivity.
However, this known compensation method cannot be used in a furnace arranged for fast heating of an object because it is not desirable to arrange a great black body in the heating chamber and because it is not possible that the black body temperature varies as fast as the object.
In order to overcome this drawback, applicant already provided another compensation method using compensation lamps as disclosed in EP-A-0458388, whereby the temperature measurement of an object is independent from the emissivity thereof. In carrying out this method, use is made of pyrometers having a narrow bandwidth and a compensation radiation is controlled as a function of the pyrometer measurement thereby to provide the exact compensation as required. That is, the mean intensity of radiation from the lamps at the pyrometer wavelength is exactly equal to the black body radiation at the temperature of the object. The drawback of the aforementioned method is that the lamps cannot be used any more to heat the object since they are directly controlled as a function of the temperature of the object.